#include "ListNode.h"
bool hasCycle(ListNode *head) {
    if (head == nullptr) {
        return false;
    }
    ListNode *slow = head;
    ListNode *fast = head;
    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) {
            return true;
        }
    }
    return false;
}